Fuel cells are increasingly being used as a power source in a wide variety of different applications. Fuel cells have also been proposed for use in electrical vehicle power plants to replace internal combustion engines. A solid-polymer-electrolyte membrane (PEM) fuel cell includes a membrane that is sandwiched between an anode and a cathode. To produce electricity through an electrochemical reaction, hydrogen (H2) is supplied to the anode and air or oxygen (O2) is supplied to the cathode.
In a first half-cell reaction, dissociation of the hydrogen (H2) at the anode generates hydrogen protons (H+) and electrons (e−). The membrane is proton conductive and dielectric. As a result, the protons are transported through the membrane while the electrons flow through an electrical load that is connected across the electrodes. In a second half-cell reaction, oxygen (O2) at the cathode reacts with protons (H+), and electrons (e−) are taken up to form water (H2O).
The main function of a fuel processor in the fuel cell system is to provide a continuous stream of hydrogen to the fuel cell stack that converts the chemical energy in the hydrogen fuel to electrical power. The fuel processor produces a reformate stream that is composed primarily of hydrogen, carbon dioxide, nitrogen, water, methane and trace amounts of carbon monoxide. During operation, the fuel cell stack demands a certain flow rate of hydrogen from the fuel processor to meet the vehicle's demand for power. The performance of the fuel processor is characterized by the flow rate of hydrogen in the reformate stream. The control of the fuel processor to maintain or track a desired flow rate of hydrogen by the fuel cell stack requires a feedback signal that measures the hydrogen flow rate. The feedback signal is used in a control algorithm to take corrective action.
Currently, there is no hydrogen sensor technology that can measure the hydrogen concentration in the reformate stream that is suitable for use in fuel cell applications. For example, existing hydrogen sensors, such as those formed from a thin film of palladium, cannot be used in the presence of water droplets that are present in the reformate stream. Therefore, the thin film palladium sensors require a water filter that slows the response time. Furthermore, oxide-based sensors such as ZrO2 and SnO2 need to operate in an oxidizing environment. The reformate, on the other hand, is a reducing environment that lacks oxygen. Thermal conductance sensors have also been proposed. However, these sensors cannot withstand the flowrates that are encountered in a fuel cell (typically 20 g/s of flow). Proton exchange membrane (PEM) and metal hydride sensors have also been proposed but none are currently commercially available.